Magnetic core



106. cdmPosmous,

COATING R PLASTIC Patented Jan. 11, 1938 UNITED STATES CROSREFERENCE PATIENT OFFICE MAGNETIC COBB Adolph F. Bandur, Berwyn, IlL, assigncr to Westcrn Electric Company, Incorporated, New York, N. Y.. a corporation of New York I No Drawing. Applica tion February 16, 1934,

Serial No. 711,552 7 Claim. (01175-21) This invention relates to magnetic cores made magnetic particles are then mixed with approxii'rom finely divided and insulated magnetic particles, and methods for making such cores.

.Objects of the invention are to provide cores made of finely divided magnetic particles having high permeabilities and low energy losses and efficient methods of making such cores.

In accordance with one embodiment of the invention, finely divided magnetic material such as finely divided particles of nickel iron allo' are mixed with talc and annealed after which an i9 sulating c0555? 01' a metallic hydroxide, sodium nely divided refracTory "metallic silicate and a silicate is applied and cores formed of t He insulated parts.

relatively thin slabs and quenched to produce a fine crystalline structure, which is .very desirable since the disintegration of the material takes place at the crystal boundaries and consequently the smaller the size of the crystals.

the finer the 'dust which can be produced from the finished product. The -rnagnetic alloy thus received in slab form is reduced to a finely divided form or dust, in any well known manner, as or 4 example, in a hammer mill or other suitable reducing apparatus and subsequently pulverized in an attrition mill.

The dust from the attrition mill is sifted and passed through a 300 mesh sieve In the case where the magnetic material in the core is a magnetic alloy including nickel and iron it has been found desirable to subject the magnetic particles to a heat treatment at a high temperature to remove the strains introduced in the magnetic material by the grinding operation which tends to,.impair the magnetic properties of the magnetic alloy in the core. The temperature of this heat treatment should be from 1500' to 1600' F. or higher. Howeve when the magnetic particles are heated to this temperature they tend to sinter to such an extent that further grinding is required which would again set a 0 mately .30 percent by weight of the roasted talc from 1500' to 1600 1''. During this process it appears that the fine particles of talc are imbedded and firmly adhere to the surface of the magnetic particles so as to substantially prevent sintering during the annealing operation. Any slight cohesion between the particles may be readily broken up by a light grinding of the material which is not sufilcient to impair the magnetic properties of the particles.

The particles are then insulated with an insulatin com sition comprising .066 percen Egnesium hyEBHde (milk of ma esia), .32 fircent sodium sficate. and .93 percent talc by weight of the magnetic material. This insulation is preferably applied insggeral coatings. For the first coating a solution is made of about 15 percen 'o w r .021 percent magnesium hydroxide and .11 percent of sodium silicate.

'Si'ritt a quantity of talc has mesay'been added to the magnetic particles to prevent slntering in g an initial set in the insulating material so that it will not be redissolved in the application of subsequent coatings. The second coat .is applied by forming a solution 0 a u percent water, .015 percent of magnesium hydroxide, .07 percefitof sodium silicate, and .21 percent of talc.

The particles are then again heated to and set the coating and two additional coa ngs are applied in the same manner, making a total of four coatin s in which approximately .066 percentcTmagnesium hydroxide, .32 percent of sodium silicate, and .93 percent of talc by weight of the magnetic material, are used. The percentages are based on the weight of the solids or non-volatileconstituents of the ingredients, and the amount of insulating material may be varied in accordance with the'fineness of the magnetic particles to be insulated.

After the dust has been insulated as described,

the magnetic material is formed into cores by subjecting it to a pressure in the neighborhood of 200,000 pounds per square inch. During the ap-' plication of this pressure the magnetic particles are again subjected to strains which impair the magnetic properties thereof and therefore the cores are again subjected to an annealing heat treatment by heating the cores Frrenmy in a Examlnel than they would be in the absence of the masv10 nesium hydroxide. The magnesium hydroxide appears to be converted into very fine particles of magnesium oxide. Magnetic particles which pass through a 300 mesh sieve have an average cross sectional'dimension about 45 microns or 18 smaller. The tale which is a refractory metallic silicate may have a cross sectional dimension from 2 to 4 microns. In binding the particles of talc together to form an insulating film over the magnetic particles, the sodium silicate appears to form 20 the chief binder and the magnesium oxide being much finer fills in the interstices to form a dense film which will not be disrupted under the high pressure used in forming the core. It has been found that voltages as high as 100 volts may be 25 applied to opposite sides of a core insulated in this manner without breaking down the insulation.

drat such as bismuth, aluminum, zinc and ca um hydrates may be used 'irTFlace oi magnesium hydrate. It'is also posasal m and ma nesium silicates may 5? ma. It 1's alfi possible to use other alfali metal silicates for the fivention sodium silicate is used.

The insulating material described above is particularly useful in insulating very fine magnetic particles. On account of the thin and tenacious 50 film which is formed over the particles, the amount of insulating material used constitutes only a small fraction by weight of the core, which results in a high permeability of the core and the eflectiveness of the insulation maintains the eddy current losses in the core at'a low value.

What is claimed is: e

1. A method ofmaking a magnetic body which comprises applying to finely divided magnetic sible to use ma esium sulphate which, due to the presence of the sodium Elicat'e, will be converted 49 silicate. Other refractory metallic silicates such' cate or a mixture of sodium silicate sucH as for instance Bolas-J um, silicate although in the preferred form 0 particles an insulating coating oi a finely divided rei'ractorymetallic silicate, magnesium hydroxide and sodium silicate, and heat treating the coating to cure the insulation.

2. A method of making a magnetic body which comprises applying to finely divided magnetic particles an insulating coating of magnesium silicate, magnesium hydroxide and sodium silicate, and heat treating the coating to.cure the insulation.

3. A method of making a magnetic body which comprises applying to finely divided magnetic Particles an insulating coating of finely divided reiractory insulating material, magnesium hydroxide and an alkali metal silicate, and heat treating the coating to cure the insulation.

4. A method of making a magnetic body of finely divided magnetic particles which comprises mixing finely divided magnesium silicate with the particles to prevent sintering, annealing the particles, applying an aqueous solution of magnesium hydroxide and sodium silicate to the particles, evaporating the solution to dryness to form an insulating coating on the particles, applying a second aqueous solution of magnesium hydroxide, sodium silicate, andmagnesium silicate to the particles, evaporating the solution to dryness,

forming a core of the insulated particles, and heating the core.

5. A method of making a magnetic body of finely divided magnetic particles which comprises applying an insulating coating of substantially .066 percent magnesium hydroxide, .32 percent of sodium silicate, and .93 percent of talc, forming cores of the particles, and heat treating the core.

6. A method of making a magnetic body of finely divided magnetic particles which comprises adding .3 percent of tale to the particles to prevent sintering, annealing the particles at a temperature in the neighborhood of 1600 F., adding an aqueous solution of .021 percent magnesium hydroxide, .11 percent of sodium silicate, evaporating the solution to dryness, adding a second aqueous solution .015 percent magnesium hydroxide, .07 percent sodium silicate, and .21 percent of talc, evaporating the solution to dryness, adding two additional solutions similar to the second solution and evaporating each of said solutions to dryness, compressing theparticles into a core and heat treating the core at a high temperature to improve the magnetic characteristics of the magnetic material therein.

'I. A magnetic structure comprising particles of magnetic material separated by insulation comprising the end products of a heat treated mixture of magnesium hydroxide, an alkali metal silicate and a filler.

' ADOLPH F. B ANDUR. 

